The effect of nitrogen on crystallisation products of Y and Er- SiAlON glasses

MSiAlON glasses with compositions of M:Si:Al = 3.45:3:2 (M= Y, Er) and varying nitrogen contents (N=5, 8, 15, and 22 equivalent %) were produced by melting appropriate powder J mixtures under nitrogen at 1715°C. Glass-ceramics were obtained by heat treatment of the glasses for 10 hours under a nitrogen atmosphere in the range 950-1200°C. From previous investigations, these compositions are expected to produce either B-phase (M₃SiAlO₅N) or I_w-phase (M₃Si ₂Al₁(O,N¹⁰) (i.e. 10 e/o N) (M= Y, Er) after heat treatment, depending on N content and temperature. Parent glasses and the resulting glass-ceramics were characterised by X-ray diffraction (XRD). The results showed that with low nitrogen containing starting compositions, it is possible to obtain XRD patterns which appear to have similar peak positions to B-phase at lower temperatures (e.g. 975 - 1025°C). However, more detailed investigations show that the peak positions in the pattern obtained do not quite coincide with the original B-phase and the phase formed has a contraction in the a dimension and an elongation in the c dimension. Crystallographic calculations show that this phase has a hexagonal unit cell with parameters of a = 3.77-3.82 Å and c = 9.83-9.97 Å. These parameters are functions of the nitrogen content of the B-phase.